opkg: add support for SHA256 verification

based on a patch by Evan Hunt <each@isc.org>
Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38302 3c298f89-4303-0410-b956-a3cf2f4a3e73